The present invention relates to manual pipettes and more particularly to an improved manual pipette including a velocity governor, "home" position latch and trigger release.
Certain commercially available single channel manual pipettes are illustrated and described in U.S. Pat. Nos. 3,827,305 and 4,909,991 by way of example. Each such pipette includes an elongated hand holdable pipette body housing an upwardly spring biased plunger unit. The plunger unit is supported for axial movement in the pipette body between a first or upper stop position in which an end portion of the plunger extends from an upper end of the pipette body. A pipette user grips the pipette body with his or her thumb over the exposed end portion of the plunger. Downward thumb action on the plunger moves the plunger downward from its upper stop position against the upward bias of a return spring to a second or lower stop position at which all fluid is expelled from a tip secured to the pipette. Between the upper and lower stop positions is a "home" position. The "home" position is defined by a "soft" stop.
In the commercially available pipettes described in the foregoing patents, the "soft" stop is defined by a second relatively stiff spring mechanism within the pipette body which is activated when the plunger unit reaches the "home" position. As the pipette user depresses the plunger unit by pressing downwardly with his thumb on the exposed end of the plunger, he can "feel" the activation of the second spring assembly opposing further downward movement of the plunger unit. U.S. Pat. No. 4,041,764, describes a magnetic detent which is engaged between an upper stop and a "home" position for a pipette piston and is disengaged by the pipette user exerting an increased axial force on a push button when it is desired to move the piston beyond the "home" position against the force of a return spring. German patent applications 239 539 A1 and 239 540 A1 describe pipettes with magnetic detents at lower stops which are overcome and disengaged by action of a return spring. In all such pipettes, the pipette user is required to continuously apply a steady downward force with his thumb to maintain the pipette plunger in its "home" position ready for emersion of a tip of the pipette into a fluid to be drawn into the tip by controlled upward movement of the plunger from the "home" position to the upper stop position.
Most commercially available multi-channel manual pipettes function in substantially the same manner as the single channel manual pipettes except that they contain multiple liquid ends each having a separate piston driven in unison from a common spring biased plunger unit. Other commercially available multi-channel manual pipettes such as the Costar Octapette and 12-Pette multichannel pipettes comprise a pistol grip structure for hand gripping by a user with his forefinger extending forward to engage and wrap partially around a spring biased actuator. In the Octapette, rearward movement of the spring biased actuator is translated into a vertical movement of a plunger unit common to the pistons in the liquid ends of the multi-channel pipette. In both types of multi-channel manual pipettes, the "home" position for the plunger unit is defined by a "soft" stop as in the previously described single channel manual pipettes and a steady force on the actuator is required to maintain the plunger unit at a "home" position.
With such conventional manual pipettes, in order obtain repeatability of operation, it is mandatory that the user return the plunger unit to its "home" position at start of each pipette operation and hold the pipette in "home" position while he immerses the tip in the fluid to be drawn into the pipette. He must then manually control the rate of return of the plunger unit to the first or upper stop position in a repeatable manner for each pipette operation in order that the same desired volume of fluid will be drawn into the pipette tips during each repeated operation. This places substantial physical and mental strain upon the pipette user over the course of a series of pipette operations wherein repeatability of operation is essential. In extreme cases the physical hand and wrist strain associated with extensive and prolonged manual pipette operation can contribute to or produce carpel tunnel syndrome.
While a pipette having a controlled rate of return has previously been developed for the assignee of the present invention and described in U.S. Pat. No. 4,763,535 and while a pipette having an attenuated rate of upward piston movement as it leaves a "home" position is described in German Offenlegungsschrift DE 39 03 241A1, there remains a substantial need for an improved manual pipette which is simple in design and operation and which does not present either a physical or mental strain to a pipette user to maintain the pipette in its "home" position and manually control the rate of plunger return to ensure repeatability of pipette operation. The present invention satisfies that need.